1. Field of the Invention
The present invention relates to an expansion dowel having an anchor bolt and an expansion sleeve. The anchor bolt has load application means provided at a first end region of the anchor bolt, and an expansion part provided at a second free end region of the anchor bolt opposite the first end region. The expansion part has a conical section widening toward the second free end region of the anchor bolt for expanding the expansion sleeve. The expansion sleeve has an inner profile with a region having a surface roughness.
2. Description of the Prior Art
Expansion dowels of the type described above serve as attachment elements for attaching objects to constructional components. In a constructional component, a borehole is formed into which subsequently an expansion dowel is driven in. Then, e.g., with a nut, which is screwed on the load application means, the anchor bolt is pulled in a direction opposite the setting direction of the dowel. As a result of the backward movement of the expansion part of the anchor dowel, the expansion sleeve is widened by the expansion part, anchoring the dowel in the constructional component.
The expansion forces, which are generated by a dowel, should be sufficiently large in order to insure the transmission of the operational load of the dowel. On the other hand, the expansion forces should not be too large, to an extent that the expansion forces negatively influence the axial and edge spacing of several expansion dowels. The expansion behavior of an expansion dowel is influenced by a geometrical shape of the expansion dowel, e.g., by selection of a cone angle of the conical section of the expansion part or by dimensions of the expansion sleeve. The geometrical shape of an expansion dowel defines a torque with which a predetermined expansion force and, thereby, a predetermined withdrawal load are achieved.
In practice, however, often higher torques than predetermined ones are applied to an expansion dowel because, in most cases, torque wrenches are not used for anchoring an expansion dowel in a constructional component. The possibility of using higher, than predetermined torques requires that the axis and edge distances be determined with a corresponding reliability with respect to the torque, otherwise a technical potential of an expansion dowel cannot be completely utilized. Further, in an extreme case, the expansion part can be pulled through the expansion sleeve which would result in a complete failure of the obtained attachment.
German Publication DE 41 23 353 A1 discloses an expansion dowel having an anchor bolt and an expansion sleeve. The anchor bolt has load application means provided at a first end region of the anchor bolt, and an expansion part provided at a second free end region of the anchor bolt opposite the first end region. The expansion part has a conical section widening toward the second free end region of the anchor bolt for expanding the expansion sleeve. The expansion part has a smooth surface and a surface provided with circumferential grooves having an increasing groove depth With an increase of friction upon locking of the dowel, the pulling of the expansion part through the expansion sleeve is prevented even with application of high loads.
The drawback of the expansion dowel of DE 41 23 353 A1 consists in that for forming the expansion part an additional and expensive operational process is required. Moreover, with the arrangement of the grooves on the conical section, sliding of the sleeve becomes difficult not only in the expanded condition of the sleeve but also at the start of the sleeve expanding process. However, it is at the start of the expanding process that the expansion sleeve should be able to slide easily to insure friction between the outer surface of the sleeve and the wall of a borehole. If at the start of an expansion process, the friction inside the expansion sleeve is greater than the outer friction, no relative movement between the expansion sleeve and the expansion part takes place, only withdrawal of the anchoring portion or even of the entire dowel occurs. Further, the grooves, which are formed on the expansion part, have only a limited active region. Upon tightening of the expansion dowel, the grooves are filled with the shaved material of the expansion sleeve so that after a certain expansion path, only thickening of the expansion part still produces a braking effect. As a result, upon application a further torque, the expansion part, without means for increasing the pulling resistance, can be pulled through the expansion sleeve.
For another purpose, namely, in order to reduce the friction between the expansion part and the expansion sleeve, European Patent EP 0 790 741 B1 discloses an expansion dowel in which the inner profile of the expansion sleeve has a region with a surface roughness. The increase roughness is produced by spark machining or with an embossing tool having a spark-machined or etched surface. The resulting diminished bearing surface of the inner profile of the expansion sleeve, along which the anchor bolt is displaced, permits to reduce friction between the anchor bolt and the expansion sleeve during setting of the expansion dowel, which reduces the force necessary for expanding the expansions sleeve with the expansion part.
The drawback of the dowel of European Patent EP 0 790 741 B1 consists in that the expansion part can be pulled through the expansion sleeve when the predetermined torque is exceeded. Further, with this expansion dowel, with an actual torque exceeding the predetermined torque, expansion forces can be generated in the constructional component which requires determination of minimal axis and edge distances with a corresponding reliability, so that the technical potential of the expansion dowel can be completely utilized.
Accordingly, an object of the present invention is an expansion dowel having a high resistance against the expansion part of the anchor bolt being pulled through the expansion sleeve.
Another object of the present invention is an expansion dowel that can be easily manufactured.